The present invention relates to an electronic switching device such as an inductive, capacitive or optoelectronic proximity switch, having an externally influenced detector which may take the form of an oscillator with a switching amplifier, and having an electronic switch such as a transistor, thyristor or triac, connected to the switching amplifier output of the detector, and also having a status indicator, the detector changes the switching state of the electronic switch when the output of the detector crosses a predetermined threshold, and the detector or the electronic switch changes the state of the status indicator in response.
Electronic switching devices of this type are designed as contactless devices and have been used increasingly for about 20 years now in place of electric, mechanically actuated switching devices that are designed with contacts. These devices are particularly useful in electric or electronic measuring, control and regulator circuits. Such proximity switches generate an indication when an influence element to which the proximity switch is sensitive, such as steel or a magnet, has come sufficiently close to the proximity switch. If an influence element for which the proximity switch is sensitive comes sufficiently close to the proximity switch, then the detector of the proximity switch changes the state of the electronic switch. If the switching device is designed as a normally open switch, the nonconducting electronic switch becomes conducting, and if the switching device is designed as a normally closed switch, the conducting electronic switch becomes blocked. With switching devices of the type being discussed, it can also be determined whether the physical quantity of influence material in proximity to the switch exceeds or falls below a suitable value.
Thus an essential component of electronic switching devices of the above-described type is, among other things, an externally influenced detector.
For example, an oscillator that can be influenced inductively or capacitively can be used as a detector to make an inductive or capacitive proximity switch, respectively. A photoresistor, a photodiode or a phototransistor can also be used as a detector to form an optoelectronic proximity switch. Finally, a temperature measuring circuit can be used as a detector to make a flow controller.
With inductive proximity switches, as long as a metal part has not yet reached a predetermined distance, K.times.V=1 for the oscillator, with K=feedback factor and V=gain of the oscillator, so that the oscillator oscillates. If the corresponding metal part reaches the predetermined distance, then the increasing attenuation of the oscillator leads to a decrease in gain V, and the amplitude of the oscillator's output decreases so that the oscillator ceases to oscillate. With capacitive proximity switches, as long as the capacitance of a sensing element is less than the capacitance between a sensing electrode and another electrode, K.times.V is less than 1 and the oscillator does not oscillate. When the sensing element reaches the predetermined distance, then the increasing capacitance between the response electrode and the other electrode leads to an increase in feedback factor K, so that K.times.V becomes equal to 1, and the oscillator begins to oscillate. In either type of switch--the inductive proximity switch and capacitive proximity switch--the electronic switch, such as a transistor, thyristor or triac, is controlled as a function of the state of the oscillator.
Optoelectronic proximity switches include an optotransmitter and an optoreceiver and detect the passage of light. The two major classes of optoelectronic proximity switches are (1) The type in which the optotransmitter and the optoreceiver are placed on opposite sides of a path to be monitored, and (2) a type in which the optotransmitter and optoreceiver are placed on the same end of a path to be monitored, while a reflector placed on the other end of the path to be monitored reflects the light beam emanating from the optotransmitter back to the optoreceiver. In both cases, the detector responds when the light beam that normally goes from the optotransmitter to the optoreceiver is interrupted by an influence element in the path to be monitored. Some optoelectronic proximity switches of the second type use a suitable influence element itself to reflect the light beam emanating from the optotransmitter back to the optoreceiver.
Another essential component of electronic switching devices of the initially and above-described type is the state indicator, by which various influence states of the detector or various switching states of the electronic switch are indicated.
Some electronic contactless switching devices have a state indicator, such as an LED, which supplies only an indication of one of two states: "below threshold" or "threshold exceeded." The state indicator is generally connected to the switching device so that it indicates whether the electronic switch is turned off or conducting.
With electronic switching devices of the type being discussed, the switching threshold generally cannot be determined independently of ambient influences. Rather, starting from a value desired and determined more or less precisely during manufacturing, the switching threshold will change according to ambient conditions, such as temperature, humidity, and contamination of lenses in the case of optoelectronic devices. A certain operating range immediately below and above the theoretical switching point must therefore be regarded as an "unsafe range." The state assumed by the device when the level of the external stimuli is in the "unsafe range" is unpredictable because the state assumed depends on ambient conditions.